In the following, the distal end of an injection device or drive mechanism is referred to as the end where a cartridge and e.g. a needle are located, whereas the opposite end is the proximal end. A dose button may be provided at the proximal end.
The general function of a drive mechanism as defined above is to set a dose and to subsequently dispense the set dose. Dose setting (dose dialling) usually requires a user to manipulate one element of the drive mechanism, preferably to rotate a dial member e.g. via a dial grip. During dose dispensing the dial member may move, e.g. rotate, back to its original position wherein a drive member, which is not actuated during dose setting is moved together with the dial member during dose dispensing. The movement of the drive member may be a rotation, a displacement in the distal direction or a combined movement e.g. along a helical path. The drive member may act on a piston rod, e.g. a lead screw, for expelling medicament from a cartridge during dose dispensing.
In addition to this basic function of a drive mechanism it is in some cases preferred to allow a resetting of an already set dose, i.e. a correction or a deselecting of a dose.
Preferably the user simply has to rotate the dial member, e.g. via a dial grip, in the opposite direction compared to the rotation during dose setting. Preferably, the drive member is not actuated during dose resetting, either.
At the beginning of dose setting, the mechanism is usually in a zero dose position, i.e. the previous dose has been fully administered and no new dose has been dialed. The user may set a dose up to a maximum dose which is defined by the mechanism, for example by providing an end stop which prevents setting of a higher dose. Typically, a maximum settable dose is 60, 80, 100 or 120 units of a medicament. During dose resetting, an already set dose may be reduced down to the zero dose position of the device. It is important that a user fully dispenses the required set dose to avoid an underdose which may have serious medical consequences. Thus, it is required to indicate to a user that the mechanism is in its zero dose position after dose dispensing.
To allow rotation of components of the drive mechanism, it is preferred if the components are mainly located concentrically about a common longitudinal axis of the drive mechanism. Thus, the components may have a tubular or sleeve-like shape. For example, the drive member and a dial or dose setting member may each be a tubular element. Some components may be provided surrounding other components fully or partly or may be provided one behind another.
An injection device comprising a housing, a piston rod which is in threaded engagement with a housing insert and a drive member is known from WO 99/38554 A1. During dose setting the drive member does not rotate relative to the housing. However, during dose dispensing a relative rotation of the drive member with respect to the housing is allowed. The drive member engages the piston rod such that a rotation of the drive member causes an axial movement of the piston rod. A ratchet is provided between the drive member and the housing insert allowing rotation of the drive member in only one direction.
A further injection device is known from EP 1 974 761 B1 wherein during dose setting, dose resetting and dose dispensing a dose grip and a dose dial sleeve rotate with respect to a housing and a housing insert between a zero dose position and a maximum dose position. A drive sleeve is provided with a clutch which is arranged such that a relative rotation of the drive member about an axis of rotation with respect to the housing is allowed during dose setting and is prevented during dose dispensing.